(a) Field of the Invention
The present invention relates to a display device and a driving method thereof.
(b) Description of the Related Art
Lightweight and thin personal computers and televisions sets require lightweight and thin display devices. Flat panel displays which satisfying these requirements are being substituted for conventional cathode ray tubes (“CRT”).
Flat panel displays used for this purpose include liquid crystal displays (“LCD”), field emission displays (“FED”), organic light emitting diode (“OLED”) displays, plasma display panels (“PDP”) and various other types of displays.
Generally, an active matrix flat panel display includes a plurality of pixels arranged in a matrix, and it displays images using thin film transistors (“TFTs”) to control the luminance of the pixels based on given luminance information.
An OLED display is a self-emissive display device that displays images by electrically exciting a light emitting organic material, and it has low power consumption, a wide viewing angle and fast response time, thereby being advantageous for displaying moving images.
An OLED display may be categorized as a top-emission type display wherein the OLED emits light to an outside through a common electrode. OLED displays may also be categorized as bottom-emission type displays wherein the OLED emits light to an outside through a pixel electrode and an insulating substrate.
A pixel of an OLED display includes an OLED and a driving thin film transistor. The OLED emits light having an intensity that depends on the current driven by the driving TFT, which in turn depends on the threshold voltage of the driving TFT and the voltage between the gate and the source electrodes of the driving TFT.
A TFT may include polysilicon or amorphous silicon. A polysilicon TFT has several advantages, but it also has disadvantages such as the complexity of manufacturing polysilicon, which thereby increases the manufacturing costs. In addition, it is difficult to make a large OLED display employing polysilicon.
On the other hand, an amorphous silicon TFT is easily applicable to a large OLED display and is manufactured by a process with fewer steps than that required for a polysilicon TFT. However, the threshold voltage of the amorphous silicon TFT shifts over time, due to an extended application of a unidirectional voltage to a gate of the TFT. This results in non-uniform current flowing in the OLED, and degraded image quality and a shortened lifetime of the OLED.
Thereby, even though the same data voltages are applied to the driving transistor, output currents from the driving transistors may differ from each other, which causes image degradation of the OLED display. For example, if a first driving transistor has a turn-on voltage that has shifted to 2 volts and a second driving transistor has a turn-on voltage that has shifted to 5 volts an applied voltage of 4 volts would cause the first driving transistor to turn on a first OLED, but the applied voltage would not be sufficient for the second driving transistor to turn on the second OLED. In order to prevent the degradation of the threshold voltage of the driving transistor, it is suggested that a reverse bias voltage be applied to the driving transistor for a predetermined time. The application of a reverse bias voltage will reduce or effectively prevent the degradation due to an extended application of a unidirectional voltage to the gate of the TFT.
When the reverse bias voltage is applied from a data driver through a switching transistor to the driving transistor, a gate-source voltage Vgs of the switching transistor decreases to cause a leakage current, and thereby changes the reverse bias voltage applied to the driving transistor.